The present Invention relates, in general, to a novel low lasing threshold deep diffused stripe semiconductor laser and, more particularly, to a novel low lasing threshold deep zinc-diffused narrow stripe AlGaAs/GaAs double-heterostructure laser.
Some of the characteristics of laser diodes that are important for fiber optic transmitter system applications include low cw laser threshold current, single mode operation, high differential quantum efficiency, small temperature dependence of laser threshold, and high output power. In addition, it is desirable that the growth of the structure and subsequent device processing be as simple as possible. Each of these desirable characteristics has been obtained separately in one or another of various laser structures which require specific control in growth or processing. However, no prior art device has combined all of these desirable characteristics in a single structure. The present Invention provides a novel solution to this problem.
In a stripe-geometry (Al,Ga)As double-heterostructure (DH) injection laser the electromagnetic field is guided along the active layer by spatial variations in both the gain and refractive index. In these devices the slightest asymmetry in the lateral dielectric constant profile can cause a shift in the lateral transverse mode toward the stripe edge where the gain is small or negative resulting in mode loss and undesirable non-linearities in the current verses light output curves. This problem can be partially avoided by reducing the stripe width to 10.mu.m or less. Unfortunately, other problems exist with such structures.
Introducing built-in refractive index profiles along the active layer provides refractive index guiding and thus tends to stabilize the lateral transverse modes. For example, the buried heterostructure (BH) laser achieves this by embedding a GaAs active region in (Al,Ga)As while simultaneously producing a device having a low lasing threshold. The transverse junction stripe (TJS) laser having an active region in a GaAs p-n junction sandwiched between (Al,Ga)As layers additionally provides a refractive index profile across the junction in the active layer due to differences in conduction type and impurity concentration.
In both the buried heterostructure and the transverse junction stripe devices, the available active regions are less than a few microns which results in low maximum output power and severe structural design limitations. Also, the transverse junction stripe device exhibits an undesirably high lasing threshold.
Another device which produces stabilized lateral transverse mode lasing is a planar stripe laser device with a deep zinc diffusion called a deep zinc diffusion stripe (DDS) laser, as described by UENO and YONEZU, "Stable Transverse Mode Oscillations in Planar Stripe Laser With Deep Zn Diffusion", IEEE Journal of Quantum Electronics, Vol. QE-15, No. 10, October 1979. This device differs from other planar stripe devices primarily in the depth of the zinc diffusion front in relation to the active layer.
FIG. 1 illustrates a typical prior art deep diffusion stripe device in cross section. The device includes an n-type GaAs active layer located between two n-type AlGaAs confinement layers on an n.sup.+ GaAs substrate. Zinc is diffused into the device to form a "U" shaped diffusion front stripe region which reaches or passes through the GaAs active layer by a few tenths of microns. As a result of the stripe, the conduction type and carrier concentrations are different in the active stripe region and the portion of the n-type active layer outside the stripe region, thus producing a built-in refractive index step along the active layer. Lasing action takes place within the active stripe region with a relatively high lasing threshold.
The present Invention provides a novel laser diode device which is similar to the deep diffusion stripe device shown in FIG. 1 with the exception of an important difference: the use of a p-type active layer in place of the n-type active layer. The present Invention provides a device having all the desirable properties of the prior art deep diffusion stripe device and, in addition, exhibits a desirable low lasing threshold. Additionally, the position of the zinc stripe diffusion front is easier to control in the present Invention thereby allowing much higher processing yields.